A piston unit of this type is e.g. shown in DE 33 12 192 A1 describing a master cylinder for a brake system of a motor vehicle. The master cylinder explained in the publication is a cylinder having two pistons arranged one behind the other, by which two separate brake systems are actuated by means of brake pressure.
It is necessary in a master cylinder (tandem master cylinder) provided with two pistons that the two pistons shut off, if possible simultaneously, two pressure chambers limited by the pistons in order that pressure will simultaneously develop in the two brake circuits. This necessitates that the two pistons have a defined distance from each other in an initial position. Said distance is defined by a captivated spring of the piston (primary piston) actuated on the inlet side. The spring of the primary piston is relatively stiff compared to a spring returning the second piston (secondary piston) so that the second spring will yield at the commencement of the movement of the two pistons and, thus, the two pistons at a predetermined distance will simultaneously close the openings that lead to a reservoir. In order to adjust the length of the first spring at the primary piston, which is defined by the position of the captivation, the above-mentioned patent application arranges for a screw having a screw head, which forms a stop for a sleeve that is movable with respect to the screw. Said sleeve, with a free end, supports a free end of the stiff spring. Another end of the spring bears against the primary piston. Readjustment of the screw renders it possible in the prior art piston unit to adjust the length of the spring. It is, however, disadvantageous, in the piston unit known from the art that both the screw and the piston must have a thread and that the screw can twist in relation to the piston.